As is well known in the art, in electrophotographic methods, there is generally adopted the method in which a photosensitive member formed of a photoconductive material such as selenium, OPC (organic semiconductor), α-Si or the like has been used to form an electrostatic latent image thereon by various means. Then, by using a magnetic brush method or the like, a toner having a polarity reverse to that of the latent image is attached onto the latent image by an electrostatic force in order to develop the latent image.
In the above developing process, there is used a two-component system developer comprising a toner and a carrier. The carrying particles called a magnetic carrier act for imparting an appropriate positive or negative electrical quantity to the toner by frictional electrification, and also act for transferring the toner through a developing sleeve containing magnets therein into a developing zone near the surface of the photosensitive member on which the latent image is formed, by using a magnetic force of the magnets.
The electrophotographic methods have been widely applied to copying machines and printers. In recent years, in the market, there is an increasing demand for high-quality images. In this technical field, with the increase in quality of images, reduction in particle size of the developer and increase in copying speed of these apparatuses are promoted, so that the stress exerted on the developer tends to become much larger. Therefore, a large problem is posed on maintaining characteristics of the developer.
With the market's requirements such as personalization and space saving, reduction in size of the electrophotographic image-forming apparatuses such as copying machines and printers is promoted. Further, with the reduction in size of the apparatuses, reduction in size of respective units used in the apparatuses have also been promoted, so that it is required to maintain good properties of the developer used therein even when using a small-size developing device, i.e., using the developer in a small amount.
In general, in order to reduce power consumption in small size apparatuses, there is a demand for a toner which is capable of being fixed to a sufficient extent even with a low fixing energy, i.e., a so-called low-temperature fixing toner. In the case where the toners can ensure a good fixing property at a low temperature, for example, by using low-molecular weight resins therein, it is possible to achieve saving of energy. However, when subjected to repeated development plural times for a long period of time, the toners tend to be spent on a surface of the carrier during continuous use under high-temperature and high-humidity conditions owing to heat or pressure generated thereupon, whereby the carrier particles tend to be strongly bonded together such that the toner is entangled between the spent portions. As a result, there tends to arise such a phenomenon that the developer suffers from blocking, etc., and variation in frictional charge amount of the developer tends to occur, resulting in variation in image density and occurrence of fogging.
In order to prevent occurrence of spent toner onto the surface of the carrier, there has been conventionally proposed the method in which the surface of the carrier is coated with various resins. For example, it is known that the surface of the respective carrier core particles is coated with a releasing resin such as a fluororesin and a silicone resin. Such a coated carrier hardly suffers from occurrence of spent toner upon the development because the surface thereof is coated with the low-surface energy material. As a result, the carrier has a stable charge amount, and the developer using the carrier exhibits a long service life.
On the other hand, the resin-coated carrier is in the form of an insulating material, and therefore hardly acts as a developing electrode, thereby causing such a phenomenon as referred to as an edge effect in particular at solid image portions. In addition, the developing bias tends to become large, so that there tends to occur deposition of the carrier on non-image portions.
In order to solve the above problem, there has been proposed the method of adjusting an electric resistance value of the coating layer by dispersing a conductive material in the coating layer. However, even when the initial electric resistance value of the carrier is adjusted by the above method, the coating layer tends to be abraded by friction, falling-off, etc., owing to stirring in the developing device during long-term use, so that if the core material is a conductive material having a low dielectric breakdown voltage, there occurs a leak phenomenon owing to exposure of the core material to outside, thereby causing such a problem that the electric resistance value of the carrier is gradually decreased and the carrier is adhered on image-forming regions.
In general, in the case where carbon black as the above conductive material is dispersed in the coating layer, the increase in amount of carbon black added tends to cause decrease in electric resistance value of the carrier. However, it may be difficult to well adjust an electric resistance of a carrier whose electric resistance value lies in a medium range of 108 to 1012 Ωcm by varying the amount of carbon black added to the coating layer.
Also, the magnetic carrier of a resin-coated type exhibits a high electric resistance value when a low voltage is applied, thereto. However, when applying a high voltage to the magnetic carrier, there tends to occur leakage of electric charges therefrom owing to influence of a core material itself thereof. In particular, when a low-electrical resistance material such as an iron powder and magnetite is used as the core material, the above tendency tends to become more remarkable. Thus, when the electric resistance value of the carrier has a high dependency on voltage, the resulting images tend to be generally deteriorated in gradation.
Hitherto, as the carrier constituting a two-component system developer, there are known an iron powder carrier, a ferrite carrier and a magnetic material-dispersed carrier prepared by dispersing magnetic particles in a binder resin.
The iron powder carrier and ferrite carrier are usually used in the form of resin-coated particles. However, since the iron powder carrier has a true specific gravity as large as 7 to 8 g/cm3, whereas the ferrite carrier has a true specific gravity as large as 4.5 to 5.5 g/cm3. Therefore, a large driving force is required to stir these carriers in the developing device, resulting in significant mechanical damage to the device, occurrence of spent toner as well as deterioration in charging property of the carrier itself, and facilitated damage to the photosensitive member. Further, since the adhesion between the surface of the particles and the coating resin is not good, the coating resin tends to be gradually peeled off during use with time, thereby causing variation in the charging property. As a result, the problems such as formation of defective images and carrier adhesion tend to be caused.
The carriers of a magnetic material-dispersed type comprising spherical composite particles formed of magnetic particles and a phenol resin as described in Japanese Patent Application Laid-Open (KOKAI) No. 2-220068 and Japanese Patent Application Laid-Open (KOKAI) No. 8-6303 have a true specific gravity of 3 to 4 g/cm3 which is smaller than those of the above iron powder and ferrite carrier, so that an energy upon impingement between the toner and carrier tends to be reduced, thereby advantageously avoiding occurrence of spent toner. Further, these carriers are far excellent in adhesion to coating resins as compared to the iron powder carrier or ferrite carrier and, therefore, hardly suffers from the problem that the coating resin is peeled-off therefrom during the use.
However, with the recent wide spread of digital copying machines and laser beam printers using a reversal development method, it has been required that the carrier has not only a high dielectric breakdown voltage owing to application of a high bias voltage thereto in the method, but also provides a developed image having a high image density and a high quality with a good gradation, etc. Therefore, the carrier is required to have a long service life capable of maintaining various properties such as charging characteristics and electric resistance for a long period of time as compared to the conventional carriers.
Further, there has been attempted some methods in which composite particles comprising ferromagnetic iron oxide fine particles and a cured phenol resin are used as a magnetic carrier for an electrophotographic developer. For example, there are known the technique of coating a surface of respective composite core particles comprising ferromagnetic fine particles and a cured phenol resin with a melamine resin to increase an electric resistance value thereof (Patent Document 1); the technique of forming a coating layer comprising a copolymer resin obtained by curing at least one resin selected from a melamine resin, an aniline resin and a urea resin, and a phenol resin, on a surface of respective composite core particles comprising iron oxide particles and a cured phenol resin to control an electric resistance value of a carrier (Patent Document 2); the magnetic carrier comprising carrier core particles comprising ferromagnetic compound particles, non-magnetic inorganic compound particles and a phenol resin and a nitrogen compound-containing or -bonding layer formed on the surface of the respective carrier core particles (Patent Document 3); the carrier comprising carrier core particles comprising magnetic particles and a binder resin, and a first resin coating layer comprising a nitrogen-containing resin and a second resin coating layer comprising conductive particles which layers are formed on the surface of the respective carrier core particles (Patent Document 4); or the like.